1. Field of the Invention
The present invention relates generally to communications systems and particularly to channel selection in a code division multiple access communication system.
2. Description of Related Art
Because the radio frequency (RF) spectrum is limited, the government, more particularly, the Federal Communications Commission (FCC), governs the use of the radio frequency spectrum. This regulation includes deciding frequency band allocation among the various industries. Since the RF spectrum is limited, only a small portion of the spectrum can be assigned to each industry. Accordingly, the assigned spectrums must be used efficiently in order to allow as many frequency users as possible to have access to the spectrum.
Because the number and size of frequency bands are limited, multiple access modulation techniques are continuously being developed and improved to improve efficiency and capacity and to maximize use of the allocated RF spectrum. Examples of such modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. CDMA modulation techniques are becoming popular because they enable more users to communicate at a given time. A spread spectrum system uses a modulation technique that distributes the transmitted signal over a wide frequency band. This frequency band is typically substantially wider than the minimum bandwidth required to transmit the signal. The spread spectrum technique is accomplished by modulating each baseband data signal to be transmitted with a unique wideband spreading code. Using this technique a signal having a bandwidth of only a few kilohertz can be spread over a bandwidth of more than a megahertz. A form of frequency diversity is obtained by spreading the transmitted signal over a wide frequency range. Since only 200-300 kHz of a signal is typically affected by a frequency selective fade (interference), the remaining spectrum of the transmitted signal is unaffected. A receiver that receives the spread spectrum signal, therefore, will be affected less by the fade condition.
In a CDMA telephone system, multiple signals are transmitted at the same frequency. A particular receiver then determines which signal is intended for that receiver by the unique spreading code in the signal. The signals at that frequency without the particular spreading code intended for that particular receiver appear as noise to the receiver and are ignored.
New generation CDMA communication networks are being formed to facilitate the transmission of large amounts of data on an as needed basis. Accordingly, a fundamental channel set is defined for transmitting on going communications between the base station transceiver systems and the mobile stations. Additionally, supplemental channels are being defined to transmit large amounts of data to a mobile station for use as needed. Because CDMA systems typically include the transmission of the communications signals from a plurality of base station transceiver systems to a given mobile station, significant amounts of resources are consumed especially when a plurality of supplemental channels are being used by a plurality of base station transceiver systems to deliver large volumes of data to the mobile station. In particular, because the supplemental channels are formed to be able to carry large amounts of data, there is a need for efficiently reserving capacity for supplemental channel data transmission only from the base station transceiver systems that are transmitting the signals to the mobile station the most clearly. By reducing the number of base station transceiver systems that transmit data over a supplemental channel to a mobile station, resources in other base station transceiver systems are not wasted and may be used for other purposes. Accordingly, there is a need for efficiently and effectively selecting the base station transceiver systems that best transmit data over the supplemental channels.
The present system and method of use comprises a system that solves the aforementioned problems by efficiently determining the optimal set of base station transceiver systems (BTSs) that are to transmit data over supplemental channels to a mobile station. More specifically, the invention includes having the mobile station transmit periodic signal strength measurement message to the BTSs to rank the pilot signal strengths being received from the plurality of base station transceiver systems. Whenever the number of fundamental channels that are active exceeds the maximum allowable number of active supplemental channels, the periodic pilot strength measurement message are transmitted by the mobile station. However, to improve resource usage, the periodic pilot strength measurement messages are not transmitted as often as required to insure that the active set of supplemental channels are the ones receiving the strongest signals. Rather, a combination of periodic pilot strength measurement messages and calculated reverse link signal strength over spectral noise density values are used to determine which BTSs should be used for the active set of supplemental channels.
More specifically, if less than a specified amount of time has elapsed since the last pilot strength measurement message was received by a BTS from a mobile station, then the pilot strength measurement message is used to rank the supplemental channels and corresponding BTSs. If, however, more than a specified amount of time has elapsed, then a calculated reverse link signal to noise ratio (Eb/No) is used to rank the BTSs and corresponding and supplemental channels. Eb/No is the total measured Eb/No across all multipath and all receive antennas per sector. In one embodiment of the invention, the list of active supplemental channels is determined at the time in which a supplemental channel is first required. Accordingly, system resources are optimized in a manner that effectively defines the supplemental channels whose signals are most likely to be received clearly by the mobile station.